Why do we say, mainly in American English, that someone’s green if they’re inexperiened or naïve?
It’s actually quite simple really: it’s because they’re fresh and new, like new green plants growing in the spring. Still, there could be many other adjectives we could use instead of green in this case. I think there’s a particular significance to the way we use green here. Perhaps more than other colours, green has a greater significance than just its surface detail.